A typical gas turbine for aircraft propulsion comprises an axial fan driven by an engine core. The engine core is generally made up of one or more turbines which drive respective compressors via coaxial shafts. The fan is usually driven by an additional lower pressure turbine in the engine core. The flow handled by the fan provides a major contribution to the thrust of the engine and is discharged through a row of outlet guide vanes (OGV) and through a bypass duct, which surrounds the rear part of the engine.
The bypass duct is split into crescent shaped portions downstream of the OGV by one or more pylons (which may also be referred to as bifurcations). The pylons are needed to host structures connecting the engine to the airframe as well as connecting lines for electrical power, fuel, oil and service air, or mechanical elements driving auxiliary machinery located on the fan case and driven by a core-mounted gear box.
The presence of the pylons means that downstream of the OGVs the bypass duct is asymmetric. The fan rotates past a non-uniform flow field as a result of the asymmetry of the bypass duct and as a result of the asymmetry of the intake duct. Under certain conditions the magnitude and pattern of the flow non-uniformity in proximity of the fan may be sufficient to reduce the fan operating range and to give rise to high levels of forcing on the fan blades. This phenomenon is undesirable and provisions are needed to reduce flow non-uniformity experienced by the fan.
One method of improving flow uniformity experienced by the fan is to use OGV rows with a desirable stagger and camber pattern. Generally, a correctly selected stagger and camber pattern acts by deflecting the flow away from a leading edge of the pylon, thereby reducing the pressure rise in its proximity. However, such arrangements may not effectively accommodate strong variations in flow conditions along the span and may suffer from performance problems if the pylon is very close to the OGV row, on account of the large variations in aerodynamic conditions from one OGV passage to another.